Converters of paper products such as book covers, magazine covers, and labels apply to their finished product a protective coating for resistance to scuff, abrasion, alcohol, soap, oils and the like.
In the past, these protective coatings have been primarily solvent based using alkyd, polystyrene and nitrocellulose polymers. Other coatings have utilized UV cured epoxy acrylates or laminates of polyethylene. Problems that are encountered with these coatings are their expense, the need for special equipment such as UV lamps, and organic solvents that need to be collected as the coatings dry. In addition, some of the coatings need to be applied off-press with a roll coater and dried in long ovens. On converter equipment where water-based inks are used, a water-based coating product that can be applied on-press with no or minimal equipment change is desirable.
Some products are available that solve some of the problems, but no product is available that has a broad range of properties to solve most of the problems. For example, acrylic dispersions supply some solvent resistance, but are poor against alkali and abrasion. UV coatings can be applied on-press and provide good protection, but they are expensive and contain monomers that are irritants or sensitizers.
There is a need for a water-based coating that can be applied by conventional flexographic or gravure methods, preferably on existing equipment, and provides a good balance of protection from solvents, corrodants, abrasion and good gloss and flexibility at a competitive price.
U.S. Pat. No. 3,684,758 and 3,705,164 disclose a process for making stable aqueous polymer dispersions by subjecting vinyl monomers to radical emulsion polymerization in the presence of a stable aqueous dispersion of (a) a high molecular weight cationic polyurethane which is gel-like in character and (b) a high molecular weight polyurethane containing anionic groups, respectively. The '164 patent in Examples 9 and 10 shows the use of a monomer mixture containing a difunctional monomer, namely butadiene and butane diol dimethacrylate, respectively.
U.S. Pat. No. 4,198,330 discloses a polyurethane latex and a polymer of at least one vinyl monomer, the polymer having been obtained by free radical polymerization of the monomer in the presence of the latex and being characterized as a hard, resinous material at temperatures below about 60.degree. C. Example III shows the use of azobisisobutyronitrile (AIBN) for initiating the polymerization of the monomers.
U.S. Pat. No. 4,644,030 discloses a method for making a stable aqueous dispersion of polymeric material comprising:
(A) producing an NCO-terminated polyurethane prepolymer in the presence of inert liquid polymerizable ethylenically unsaturated monomer material, PA0 (B) dispersing the product from (A) in water, PA0 (C) chain-extending the prepolymer in the resulting aqueous dispersion, and PA0 (D) subjecting the aqueous dispersion from (C) to vinyl addition polymerization conditions to polymerize the monomer material in-situ, the weight ratio of chain extended polyurethane to monomer material being about 9:1 to about 1:9. Also disclosed in U.S. '030 are the resulting aqueous dispersions and their use for making films, sheets and other products. Example VII shows the use of the difunctional hexanediol diacrylate in the monomer material. PA0 (A) forming a solution of an anionic water-dispersible isocyanate-terminated polyurethane in at last one vinyl monomer; PA0 (B) dispering the solution in an aqueous medium; PA0 (C) chain extending the polyurethane, and either PA0 (D) adding further vinyl monomer, and PA0 (E) initiating polymerization of the vinyl monomer, or PA0 (F) initiating polymerization of the vinyl monomer, and PA0 (G) adding further vinyl monomer during polymerization. Example 13 shows a polyurethane-vinyl polymer preparation using AIBN initiator and diallylphthalate as a difunctional monomer. PA0 (a) forming a carboxy-containing, water-dispersible, isocyanate-terminated polyurethane prepolymer, PA0 (b) adding a vinyl monomer mixture which contains a polyethylenically unsaturated monomer to the prepolymer to make a prepolymer/monomer mixture, PA0 (c) adding a tertiary amine to the prepolymer/monomer mixture, PA0 (d) dispersing the prepolymer/monomer mixture in water, PA0 (e) adding an oil-soluble free radical initiator and a chain extender to the aqueous dispersion, and PA0 (f) polymerizing the vinyl monomers and completing the chain extension of the prepolymer by heating the aqueous dispersion.
EP Application 0 308 115 A2 discloses a surfactant-free aqueous polymer dispersion containing an anionic water-dispersible polyurethane and a vinyl polymer in a weight ratio of from 10:90 to 90:10, the dispersion having been obtained by: